Musical tuning instrument



H mm M a NVENTOQS O m ma w e w aim a f m I R H A M n n 60 u m M i n@ wjfkn m a]? m 8 EJ wwmxvlllinv k Patented July 9', 1940 PATENT OFFICEMUSICAL TUNING 1N STRUDIENT Kenneth N. Bergan and Wallace 0. Skarshaug,Decorah, Iowa Application August 3, 1938, Serial No. 222,765

.7 Claims. (01. 84-454) The invention relates to musical tuners and moreparticularly to devices for determining the musical pitch of aninstrument or voice or several of them.

5 One object of the present invention is to provide a musical tunerwhich may be operated entirely inaudibly to give only a visualindication of the point at which the tone being tested, from aninstrument or voice, attains a preselected pitch.

Another object of the invention is to provide a musical tuner capable ofperforming a silent pitch-indicating operation, of the general typenoted above, and embodying means for indicating visually whether thetone being tested is sharp or flat.

Another object of the invention is to provide a highly flexible musicaltuner of such character that it may be readily adapted for either silentor audible tuning, that is, tone testing operations in which a visualindication is given as to whether or not the tone being tested is on oroff pitch with respect to a selected reference frequency and inwhich-the reference frequncy can be rendered audible or inaudible atwill.

Still another object of the invention is to provide a musical tuner forindicating whether or not two or more tones being tested, such as thoseproduced simultaneously from several instruments in an orchestra orvoices in a chorus are of the same pitch.

A further object of the invention is to provide a musical tuner of thetype noted above for comparing the pitch of a plurality of tones,embodying means operable at will, not only to indicate whether or notsuch tones are in pitch with each other, but also whether or not theyare in pitch with a standard reference frequency of selected value.

A further object of the invention is to provide a novel form of beatdiscriminator circuit which is adapted for use in all apparatus of thetype indicated.

More specifically, the invention also resides in the provision of beatdiscriminator circuitembodying a triode amplifier in which the incomingsignals are subjected to grid rectification and car-' rier waveinterference is eliminated by the use of capacity coupling between theanode and cathode of the triode.

Further objects and advantages of the invention will become apparent asthe following description proceeds, taken in connection with theaccompanying drawing in which:

Figure 1 is a generally schematic wiring diatgram of a. musicaltunerembodying the invenion.

Fig. 2 is a fragmentary detail view of a portion of a cathode ray typetube utilized as an indicator in the apparatus of Fig. 1. 5

Fig. 3 is a. modified form of indicator circuit.

One of the prime virtues of the particular tuner illustrated is itsadaptability for a large variety of tone testing operations. The threeprincipal types of tuning contemplated are: first, silent tuning inwhich only a visual indication is given as to whether or not a tone ortones being tested are of a selected pitch; second, audible tuning inwhich an audible reference tone is produced by the apparatus togetherwith a visual indication'as to whether or not a tone or tones beingtested are of the same pitch as the reference tone; and, third, thetesting for coincidence of pitch between two or more tones being tested.The variety of uses to which such a flexible instrument can be put isvery large. For example, in tuning the instruments of a band ororchestra the apparatus can be conditioned to produce an audible tone ofselected pitch and then conditioned for silent tuning during which timeall of the members of the orchestra sound the same note on theirrespective instruments. The tuner will, in such case, indicate whetheror not any of the instruments are off pitch so that the necessaryretuning of the instruments can be rapidly made. U Similarly, in musicclasses, whether voice or instrument, it is frequently useful to testthe pupils sense of pitch. For this purpose the instructor can conditionthe tuner for silent tuning and then observe by it whether or not thepupil sings or plays a tone of the selected pitch. In someinstances, itmay be desirable to provide a tuner of more limited application and insuch case some of the elements may be omitted, as, for example, theaudible tone mechanism, or other alterations may be made as therequirements of a particular case dictate. Although particularembodiments of the invention have been shown and described in somedetail for purposes of illusatration of the invention, there is nointention thereby to limit the invention to such embodiments, but, onthe other hand, the appended claims are intended to cover allmodifications and alternative constructions within the spirit and scopeof the invention.

As to the identity of the general elements of apparatus illustrated, itcomprises: a reference frequency unit H], for producing electricalimpulses of predetermined frequency corresponding to a selected tonalpitch, and a test frequency unit H, which produces electrical impulsesof the same frequency as that of the tone being tested. These twoelectrical impulses of controlled frequency are amplified in a vacuumtube amplifier I2 and imposed on a beat discriminating and indicatorcircuit I4, which includes a visual indicating device l5 shown herein asa cathode ray tube of conventional form. This latter device gives avisual indication as to whether or not the tone or tones being testedare of the same frequency of fixed value produced by the reference unitIll. The indicator can also be used to show coincidence of pitch betweentwo or more tones being tested irrespective of their relation to anyfixed or reference value. Also included in the apparatus is an audibletone monitor unit it. This latter unit produces an audible tone or soundof the same pitch frequency as the fixed frequency electric currentproduced by the reference unit Ill. Electric current is supplied to thevarious elements of the apparatus from a rectifier unit ll connected toalternating current supply lines lid-L2 through an on-ofi switch it. Theentire tuner apparatus is preferably mounted as a unitary structurewithin a portable carrying case. Since the tuner is operated from anordinary alternating current supply line and requires no batteries orother auxiliary sources of current it can be readily plugged in on anyavailable lighting circuit or the like, thereby making possible its usein a home, school or auditorium, as the case may be.

Turning now to a more detailed consideration. of the various generalcomponent elements of the apparatus outlined above, the referencefrequency unit l0 may be of any desired type. By way of illustration, ithas been shown herein as an audio-frequency vacuum tube oscillatorembodying a triple grid pentode tube l9, type 6J7, and a triode tube 20,type 605. This type of oscillator is preferred because it has a highdegree of frequency stability. It will be understood, however, that alarge number of alternative oscillator circuits are available for thispurpolse. In the particular circuit shown, the pentode 69 includes theusual plate 2! indirectly heated cathode 22, as well as three grids 23,24, and 25. Similarly, the triode 20 includes a plate 26 indirectlyheated cathode 2i, and grid 28.

The triode 20 is, in general, utilized to provide a predetermined fixedvoltage supply for the grid bias on the pentode ill. The fixed voltageoutput of the triode 20 is maintained despite variations in alternatingcurrent power supply voltage which may, and in ordinary lightingcircuits frequently do, occur in the supply lines L1L2. The cathode21 ofthe triode 20 is connected to ground through a cathode bias resistor 29,and the grid 28 is connected to the cathode through a variable grid biasresistor 30. A neon lamp 3| is shunted across the resistor 30, andconnected in series with a ballast resistor 32 across a high voltagesupply line 33 and ground. It is a characteristic of neon lamps thatthey are essentially constant voltage devices or, in other words, thevoltage drop across them remains substantially constant irrespective ofvariations in the external voltage on the circuit. Accordingly, the neonlamp 3| maintains a substantially constant voltage across the grid biasresistor 30. The

cathode 21 of the triode 20 is connected to the high voltage line 33through a large bias resistor 34 while the output or plate circuit ofthe triode is connected to the cathode or input circult of the pentode19 through a conductor 35. The values of resistors 34 and 29 are sochosen that their ratio will equal the amplification factor of thetriode. When so arranged, the regulation of the tube is extremely high.

In the pentode, or oscillator circuit proper, electrodes 23 and 25 areconnected to act respectively as the anode and grid elements of anoscillator triode, with the cathode 22 forming the third element of thetriode. This triode is coupled through the electron stream with theplate 2| in the tube output circuit. A condenser 44 connects the gridand anode elements of the oscillating triode and a grid leak resistor 45connects the grid 25 to the negative or low voltage side of the circuit.An audio frequency tank or oscillator circuit is formed by an iron corechoke 43 and a plurality of condensers 4'! to 41, arranged to beselectively connected in parallel with the inductive choke by a selectorswitch 49. To complete the circuit a load resistor 4| is connectedbetween the pentode plate and cathode, a filter condenser 48 isconnected between the high and low voltage supply lines 33 and 35, abias resistor 46 is connected to the plate 2|, the bias resistor 36 andbypass c0ndenser 31 are connected in the cathode circuit, and the biason the control grid 23 is varied by a variable resistor 23 for a purposehereinafter described. To insure adequate coupling between the cathodeand plate of the pentode a low capacity condenser 42 may be used tosupplement the electron coupling. The output or plate terminal of thepentode I9 is connected to ground through load resistors 39 and 40 and acoupling condenser 38. Variations in the voltage drop through theresistor 40, caused by the oscillating output current of the pentodeoscillator, are used to control the potential bias on a control grid ofa vacuum tube included in the amplifier unit i2 for a purposehereinafter described.

In order to condition the reference frequency unit ill to produce anyone of a predetermined series of frequencies, the selector switch 49 isutilized. When in its off position, the switch 49.

by-passes all of the condensers M -41 to cut off all output from theoscillator. When shifted to its other positions, it closes correspondingcircuits through selected ones of the condensers il -41 densers are sochosen with respect to the associated circuit constants that an outputcurrent from the oscillator circuit will be obtained which has afrequency equal to that of some particular corresponding set of notes inthe diatonic scale.

Preferably, if only four condensers are provided, the circuits arearranged so that the corresponding output frequencies will be that of Bfiat below middle C, F, A, and B flat an octave above the first B fiat.The selector switch 49 is manipulated by a control knob 50 located onthe exterior of the unit. A graduated scale may be provided with thecontrol knob 50 so that the operator need only read the indicia of themusical note desired and turn the knob to the corresponding position. Ofcourse, any number of tones may be provided by utilizing a correspondingnumber of condensers of the proper values. Also, if desired, other meansof tuning such as a variable inductance could be used in some oscillatorcircuits.

In some instances it is desirable to vary the pitch of the referencefrequency slightly above or below its standard value. For example, if aThe capacities of these latter conand the indicator l shows that he isslightly off, then the reference frequency from the unit It is variedprogressively until the indicator shows that the reference frequencyexactly coincides with the musical tone being tested. Thereupon theoperator of the tuner observes whether or not it was necessary to makethe reference tone sharp or flat so as to match that of the instrumentbeing tested. To this end a vernier control knob 23 is provided forcontrolling the variable resistor 25 in the control grid circuit of thepentode i9. By changing the grid biasing potential, through the mediumof the variable resistor 23, the pitch or frequency of the oscillatoroutput is correspondingly increased or decreased. A scale associatedwith the control knob 23 shows whether the change has made the notesharp or flat. Since the output frequency of the oscillator is verysensitive to changes in'the grid bias potential it is, for that reason,desirable to utilize the triode 20 and neon lamp 3i, as previouslydescribed, to maintain a fixed or constant voltage between the high andlow voltage lines 33 and 35 across which grid bias control resistor 23is connected.

The test frequency unit 1 I serves to convert the vibrations in air,caused by the musical note or tone being tested, into pulsating electriccurrent of corresponding frequency. Any suitable microphone circuit maybe utilized for this purpose. In the particular construction shown, apermanentniagnet type loud speaker 5! has been used as a microphone andits actuating winding 52 is coupled through transformers 53 and 5! withthe control grid 55 of a duplex-diode pentode 56, type 637. The pentodesection of this tube is used as an amplifier for the microphone currentsand the diode section is used as a rectifier in the beat discriminatorcircuit H as is hereinafter described in greater detail. The tube 55embodies a cathode 51, plate 58, screen 59, suppressor 60, and diodeplates 6|. A cathode bias resistor 62 and by-pass condenser 63 connectthe cathode 5'! to ground while the by-pass resistor 84 connects thescreen 59 to the cathode. The suppressor 65 is connected directly to thecathode. The output of the amplifying pentode section of the tube 55 iscoupled through a condenser 85 with the amplifier it.

The respective audio-frequency output currents of the referencefrequency unit i0 and test frequency unit II are amplified by theamplifier unit 12 and this latter unit also embodies means forcontrolling the volume of the reference frequency unit output. Theamplifier unit i2 may conveniently include two power-amplifier tubes 66and 61, (type 41) connected in push-pull relation. 68 and 69, plates andII, control grids 12 and I3, screens 14 and 15, as well assuppressorsand 11. The cathodes are in each case connected to ground through theusual cathode bias resistor and by-pass condenser. The output of thereference frequency unit I0 is applied to the control grid 12 of thetube 56 and similarly the output of the test frequency unit is connectedto the control grid of the tube 51. The plates 10 and H of the amplifiertubes are coupled in pushpull relation through series connectedcondensers 18'|9 with their common terminals connected 'to ground. Thesecondensers by-pass to ground and high frequency distortions that mayoccur in the pentode power amplifiers. In order to control the volume ofoutput for the reference frequency unit, a variable volume controlresistor These tubes embody respective cathodes or pitch as thereference frequency-produced by v the unit iii, the amplified outputcurrents of the units l0 and Ii are imposed on a beat discriminatingcircuit it. As a preliminary to a consideration of this circuit, itshould be noted that.

when currents of two different frequencies are superimposed in a circuitthat strong beats or, in other words, high peaks in the oscillation willoccur, although at a much lower frequency than that of either of thecomponents, when the two superimposed frequencies are whole-numbermultiples of each other. In the event, however, that the two currentsare of substantially identical frequency, there will be no such beats orperiodic low frequency variations in the integrated sum of the componentfrequencies. This phenomena is utilized in the present apparatus foractuating an indicator device. To this end, an indicator is providedwhich will be actuated by any beats produced in the circuit but willremain unaffected if there are no beats. Since the absence of beatsshows that the two frequencies are of the same value or pitch, apositive indication is obtained as to whether or not the musical tonebeing tested is in pitch with the standard or reference frequency.

In the particular form of beat discriminating circuit illustrated, atransformer 82 is provided having a center-tapped primary winding 83 anda secondary winding 84. The outputs of the amplifier tubes 56-61 areconnected across respective halves of the primary winding 83. Thealternating potential appearing in the secondary winding 84 of thetransformer has a mean value which is a function of the relativefrequency of components of the primary current. In particular, if thecomponents of the primary current are all in phase so that a steady,generally sinusoidal variation in secondary potential is obtained, themean value of the secondary potential will be constant. On the otherhand, if the frequencies of the primary current components differ fromeach other, the mean value of the secondary potential will vary at afrequency corresponding to the beat frequency of the components. To putit another way the conponents of primary current are combined to form acarrrier wave of audio-frequency and the envelope of the carrier wave isdetermined by the frequency relation of the components. If thefrequencies of the components are equal the envelope will berectilinear, but if they are not equal the envelope will be aperiodically varying curve. The periodicity of the envelope is the beatfrequency? of the components.

The output of the transformer 82 is rectified by the diode section ofthe tube 56 so as to produce a direct current which pulsates at the samefrequency as alternating voltage produced in the transformer secondarywinding 84. For this purpose, one terminal of the transformer secondarywinding 84 is connected to the diode plates SI of the tube 55. The otherterminal of the transformer secondary is coupled by a condenser 85 witha control grid 86 of the cathode ray tube I5. The pulsations insecondary current due to the audio-frequency carrier wave are filteredby a network shown as including a bias resistor 9| and by-pass condenser9| connected between the transformer output and ground. Accordingly theonly variations in current or pulsations applied to the couplingcondenser are beat frequency pulsations or ripple in the direct current.The beat discriminator circuit l4 herein illustrated is so sensitivethat the indicator 15 will show even very small variations in the meanvalue of a carrier wave having a comparatively large amplitude.

Special advantages result from the use of a cathode ray tube as anindicator, as will be hereinafter pointed out in great detail. Of evengreater importance, however, are the advantageous results achieved bythe use of the novel circuit associated with the indicator. This novelcircuit is, in fact, applicable not only to cathode ray tube indicatorsbut to other types as well. The cathode ray tube ii is of conventionalform, being preferably a type 6E5 and embodies, in addition to thecontrol grid 86, an indirect y heated cathode 81, plate 88 and target89. A fluorescent screen 98 (Fig. 2) is lighted bythe electron streamand a segmental shadow or dark image 8| appears on the inner surface ofthe conical screen, the width or angle of this image being a function ofthe biasing potential on the grid 88. Changes in the width of the image8i cause the device to resemble a blinking eye. When the potential onthe grid is from five to eight volts negative with respect to thecathode, the image 9| disappears entirely, or in common engineeringparlance the eye closes.

The circuits herein shown are so arranged that the eye will remain openand steady, i. e. retain an image 9| of fixed width on the fluorescentscreen when there are no beats imposed on the beat discriminator circuitl4, and similarly, the eye will flash, i. e. the image 98 willsuccessively decrease in size until it disappears, or almost so, andthen widen out again when beats are imposed on the circuit.

The novel beat discriminator circuit M is characterized first, by theinitial rectification described above which separates most of theaudiofrequency carrier wave from the beat and second, by the use of gridrectification on a triode tube whose grid is capacity coupled with theoutput of the first rectifier. In the present instance the plate 88,grid 88 and cathode 81 of'the cathode ray indicator tube l5 itself havebeen used as the triode. but it will be understood by those skilled inthe art that a separate triode tube could be connected to a structurallyseparate indicator such as a neon lamp (Fig. 3). In the particularillustrative circuit shown (Fig. 1) the upper terminal of thetransformer secondary winding 84 is connected to ground through the biasresistor 88 and a by-pass condenser 98 for filtering purposes as waspreviously noted. Also, the grid 88 and cathode 81 are connected toground and to each other, through their respective bias resistors 92 and93. The resistors 92 and 93, which interconnected the grid 86 andcathode 81, are of particular importance since theycomplete a circuitfor grid rectification as is described more in detail below. The plate88 is connected to the high potential line 33 through a load resistor86*, the target 89 being connected to the opposite terminal of thisresistor.

When using a type 6E5 cathode ray tube it has been found desirable tomake the resistor 82 of approximately three megohms and the resistor 93of approximately two hundred ohms to achieve grid rectification. Anybeat frequency ripple in the rectified current from the transformersecondary winding 84 imposes a varying potential on the couplingcondenser 85 so that a corresponding pulsating potential appears in thecircuit of the grid 88. The grid and cathode 81 act, however, as diodesof a, rectifier. More particularly, the grid 88 can become more negativewith respect to the cathode 81, during the negative half cycles of thebeat frequency pulsations, but it cannot become more positive withrespect to the cathode because when-it tends to do so current fiowsbetween the grid and cathode so that a rise in the potential on the gridis prevented. The flow of such a current between grid and cathode is, ofcourse, only possible when the values of the resistors 92 and 83 are sochosen to permit it and, hence, it will be appreciated that theparticular values chosen for these resistors are important. The visibleeffect of such grid rectification action is to prevent the image 9| onthe screen 98 from opening up beyond a fixed width, although the imageis permitted to close. In other words, the eye can blink closed, but itcannot open beyond a predetermined amount.

By utilizing a condenser 94 of very low capacity to couple the plate 88and cathode 81 the necessity of a low pass filter to eliminate theaudio-frequency carrier wave interference effects from the plate isobviated. In other words, the coupling condenser 85 between thetransformer 82 and cathode ray tube l5 passes not only the beatfrequency pulsations, but also to some extent the audio-frequencycarrier wave pulsations. If the indicator is to respond only to beatfrequency pulsations then the effect of the audio-frequency carrier wavepulsations must be eliminated. This latter elimination function couldconceivably be performed by a low pass filter. Such a device would,however, be undesirable in the instant apparatus, since a largeinductance would need to be included in it and the circuit would be muchless sensitive to beat frequency pulsations than the improvedarrangement herein disclosed. Instead of using a low pass filter thecondenser 88 is utilized for by-passing the audio-frequency carrier waveinterference directly to ground. The capacity of this condenser ispreferably about 0.05 micro-farads.

By utilizing a cathode ray tube as the indicator for the tuner the costof the apparatus is minimized since the elements of this single tubeconstitute a triode amplifier, a diode rectifier for controlling theaction of the indicator and a visual indicator. Furthermore, such acathode ray tube is rugged in construction as compared to such measuringdevices as a galvanometer and is, moreover, very sensitive in operation.The cathode ray tube may be conveniently connected to the remainder ofthe apparatus by flexible leads so that it may be mounted on a musicrack or other convenient position for the use of a teacher or orchestraleader. Moreover, the oathode ray tube lends itself to the use of asimple condenser arrangement such as that described above, foreliminating the audio-frequency carrier wave from the indicator circuit.

An alternative indicator circuit is shown in Fig. 3. In this circuit aneon lamp H8 is utilized as a visible indicator and is operated from atriode vacuum tube II I. The circuit is almost identical with that forthe cathode ray tube I 5 and the triode Ill functions in the samegeneral manner as the triode elements of the tube 15. In particular, theplate H2, of the triode III, is connected to the high potential or +3line 33, the grid 3 is coupled to the control circuit by the condenser85, and the grid and cathode H4 are connected together and to ground bybias resistors 92 and 93. The latter complete a grid rectificationcircuit as before. The neon lamp H is connected across a droppingresistor II in the output or plate circuit of the triode and a by-passcondenser, of preferably about 0.05 micro-farads connects the plateterminal of the lamp to ground. With such an indicator circuit, any beatfrequency pulsations applied to the coupling condenser 85 cause the lampH0 to flash off and on in synchronism. This, of course, indicates thatthe frequencies being tested are off pitch just as before. tested are inpitch then, of course, there are no beat pulsations and the lamp IIOglows steadily.

The electrical impulses from the oscillator or reference frequency unitI0 may be converted into an audible sound or signal by the monitor unitI6. This tone monitor unit has been shown herein as comprising anelectro-dynamic loud speaker 95 provided with an actuating winding 96and a field winding 91 (included in the power unit IT). This loudspeaker 95 is coupled with the amplifier I2 and beat discriminatorcircuit I4 by a transformer 98 having a primary winding 99 and secondarywinding I09. One terminal of the primary winding 99 is connecteddirectly to the high voltage supply line 33 while the other terminal isconnected to one fixed contact of a selector switch It. When theselector switch I 0I is in the position shown the transformer primarywinding 99 is connected across the volume control resistor 80 so thatthe output of the amplifier tube 66 is impressed on the transformer 98and through it to the loud speaker 95. On the other hand, when theselector switch IN is shifted to its second position with the lowercontact closed the loud speaker transformer 98 is open-circuited and theoutput of the amplifier tube 56 connected directly to the discriminatingcircuit transformer 82. Hence, by the simple manipulation of theselector switch IOI the reference frequency produced by the tuner can bemade either audible or inaudible.

The power supply unit I I is of conventional form and embodies a fullwave rectifier tube I02 provided with a cathode I03 and anodes I04.Current is supplied to the rectifier tube I02 through a step-uptransformer I 05 comprising a primary winding I 06, connected acrosssupply lines Li-Lz and a second winding I01 having its end terminalsconnected to the respective vacuum tube anodes I04. A center tap on thesecondary winding I01 is grounded. Heater current for the rectifier tubecathode or filament I03 is supplied from a transformer secondary windingI08 and the high voltage output on B voltage of the rectifier tube issupplied to the various vacuum tubes in the tuner apparatus through thehigh voltage conductor 33. The A current, or heater current, for thevarious vacuum tubes in the apparatus is supplied from a low voltagetertiary winding I 09 on the transformer I05. The connections from thiswinding to the various cathode heaters have been omitted in order tosimplify the wiring diagram. A filter for the B circuit is formed byfilter condensers 91 and 91", together with the fleld winding 91 of theloud speaker 95, this winding being used as an audio-frequency choke.

The operation of the tuner illustrated is very simple in so far asmanipulatory operations as performed ,by the operator are concerned. Asin the case of an ordinary radio receiver, the in- If the frequenciesbeing ternal circuits may appear to be rather complicated but themanipulations performed by an operator are few in number and require noparticular skill. In conditioning the tuner for operation the supplylines L1L2 are first of all connected to a suitable alternating currentsupply circuit by an ordinary plug connection and the on-off switch I8is closed. Next it Will be assumed that the operator wishes to test Bfiat below middle C on some band instrument such as a horn. For suchpurpose the selector switch 50 is turned to the left until its pointerregisters with the B flat on the left side of the graduated scale andthe knob 23 is turned to its mid-position. Furthermore, the selectorswitch It is moved to its "off position if the tuning is to be silentinsofar as the tuner is concerned. The musician then sounds B flat onthe horn being tested at some point reasonably close to the microphone57. If the tone sounded on the horn is of exactly the proper pitch, thatis, B fiat, then the eye of the indicator tube I5 remains open or, inother words, the image 9| is steady. If the note sounded on the horn issharp or flat, however, the "eye of the indicator tube I5 will blink,since the frequency of the electrical impulses from the unit II willdiifer from the refference frequency output of the unit l0 and as aconsequence a beat frequency variation in potential will be applied tothe cathode ray tube control grid 86. Upon observing such a blinking ofthe visual indicator tube I5 the operator of the tuner may then turn thecontrol knob 23' slightly back and forth until the indicator tube i5stops blinking. Upon achieving such a point of balance in the input andreference frequencies the position of the control knob 23 is observed sothat the operator knows whether the note sounded on the horn was sharpor flat and approximately how much. The testing of the pitch of otherinstruments or voices is carried on in the same manner. In the eventthat a bowed stringed instrument is being tested it has been found thatit is often difilcult for anyone but a skilled player to maintain a noteof exactly constant pitch by pulling a bow across the strings.Accordingly, it is better to pluck the strings of such an instrument totune it with the tuner herein illustrated. When the string of a violinis plucked, for example, the eye of the indicator I5 blinks once andthen remains steady if the string is properly tuned to the referencefrequency for which the selector 50 is set. If the violin string is outof tune, however, the eye of the indicator I5 will continue to blinkjust as in the case of an out-of-pitch note from any other instrument. Aplurality of instruments or voices or both can be tested simultaneouslythrough following the same procedure. In such case, however, if any oneof the group is off pitch the eye of the indicator tube I5 will blink sothat they will thereafter have to be tested individually in order todetermine which instrument or voice is the offender.

In some instances it is desirable that the musicians be able to hear amonitor tone of the selected pitch in order to aid them in tuning theirinstruments or placing their voices. For this purpose the selectorswitch IOI is shifted to its "on position and thereafter the loudspeaker 95 produces a tone or signal of a pitch determined by thesetting of the selector 50. The visual indicator I5 remains inoperation, however, so that the same observations can be made as beforeas the point.

to whether or not the instruments or voices being tested are actually inpitch with the monitor tone. The adaptability of the tuner for eithersilent or audible tuning is particularly important for variousspecialized uses. For example, music teachers often find it desirable totest a pupil's sense of pitch and this can only truly be tested when atone is played or sung by the pupil in the absence of any reference toneor audible guide. Furthermore, the overtones or harmonics produced byvarious instruments are sometimes so widely different from each otherthat there is some difiiculty in determining by ear whether or not theymatch exactly the pitch of an audible reference tone. By utilizing thevisual indicator apparatus, however, there is never any doubt on Thetuner apparatus illustrated may also be utilized for a third type ofpitch testing operation in which no reference tone or im pulses areused. In this third type of operation the tones from two or moreinstruments or voices are compared directly to each other rather thanbeing compared to some reference pitch. For this type of operation theselector 5% is turned to its off position so as to disable theoscillator unit Ill. The sound waves from the several instruments orvoices being tested enter the microphone 51 and set up an input currentwhich is made up of a series of components, one for each instrument orvoice in the group. If these components are all of the same frequencythen there will be no beat frequency imposed on the indicator tubecontrol grid 86 and hence the "eye of the tube will remain steady andopen. In the event, however, that any of the tones sounded are off pitchwith respect to each other there will be a beat frequency imposed on thecontrol grid 86 and the eye of the indicator tube It) will blink. Ingeneral, it will be seen that the tuner described is highly adaptable toa large variety of pitch testing operations and that all of them may becarried out with only very simple manipulations of the controls for thetuner.

We claim as our invention:

1. A musical tuner, comprising, in combination, means for producingelectrical impulses at a frequency corresponding to the pitch of apredeterminedtone in a musical scale, means for producing periodicelectrical impulses at a frequency corresponding to the pitch of amusical tone being tested, an indicator including a cathode ray tubeembodying a control grid, and means responsive to the beat frequencybetween said two first named frequencies for controlling biasingpotential on said control grid.

2. A musical tuner comprising, in combination, means for producingelectrical impulses at a frequency corresponding to the pitch of apredetermined tone in a musical scale, means for producing periodicelectrical impulses at a frequency corresponding to the pitch of amusical tone being tested, a transformer embodying inductively coupledprimary and secondary windings, said primary winding including twoseries-connected sections, means for applying said first namedelectrical impulses to one of said primary winding sections and forapplying the second named electrical impulses to the other section ofsaid transformer primary winding, an indicator including a cathode raytube embodying a control grid, means for rectifying the potential insaid transformer of said secondary winding, and means including acapacity coupling between said transformer secondary winding and saidgrid for applying to said control grid a biasing potential varying inaccordance with the beat frequency between said two first namedfrequencies.

3. A musical tuner comprising, in combination, an input circuit, meansfor setting up in said input circuit a periodic alternating potentialmade up of two components one corresponding in frequency to the pitch ofa musical tone being tested and the other corresponding to the pitch ofa predetermined tone in a musical scale, means for rectifying thecurrent in said input circuit, a triode vacuum tube including an anodeand cathode and control grid, means including a capacity couplingbetween said input circuit and grid for applying to the latter a biasingpotential varied in accordance with variations in. the mean value of thepotential in said input circuit, means including a resistance connectedbetween said grid and cathode for establishing a diode rectificationcircuit therebetween, and an indicator device actuatable in response tovariations in the electron stream flowing to said plate under thecontrol of said grid.

4. In an apparatus for indicating the presence of low frequencyvariations in the mean value of an oscillatory carrier wave, thecombination of a triode vacuum tube embodying a cathode and an anode anda. control grid, an input circuit, a capacity coupling between saidinput circuit and said control grid, means including a resistanceconnection between said grid and cathode for establishing a dioderectification circuit therebetween, and an indicator device actuatablein response to variations in the electron stream flowing to said plateunder the control of said rid.

5. In an apparatus for indicating the presence of low frequencyvariations in the mean value of a periodic carrier wave, the combinationof a cathode ray tube embodying a cathode and an anode and a controlgrid and an image screen, the size of the image on said screen being afunction of the variations in grid potential from a selected value, aninput circuit, a capacity coupling between said input circuit and saidcontrol grid, and means including a resistance connection betweensaid'grid and cathode for establishing a diode rectification circuittherebetween to prevent the potential in said grid from varying in morethan one sense from said selected value.

6. In an apparatus for indicating the presence of low frequencyvariations in the mean value of an audio-frequency carrier current, thecombination of a. triode vacuum tube embodying a cathode and anode andcontrol grid, an input circuit, a coupling between said input circuitand said control grid, an indicator device actuated in response tovariations in the electron stream flowing to said plate under thecontrol of said grid, and means including a capacitive coupling betweensaid plate and ground for by-passing any audio-frequency interferencefrom said plate directlyto ground.

7. In an apparatus for indicating the presence of low frequencyvariations in the mean value of an audio-frequency carrier current, thecombination of a cathode ray tube embodying a cathode and anode andcontrol grid and image screen, an input circuit, a capacitive couplingbetween said input circuit and said control grid, and

means including a low capacity coupling between said plate and groundfor by-passing any audiofrequency interference from said plate directlyto ground.

IENNETH N. BERGAN.

W. OLIVER SKARSHAUG.

